Volkswagen Front Wheel Drive 1974-1989 Repair Guide

Checking Engine Compression

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A noticeable lack of engine power, excessive oil consumption and/or poor fuel mileage measured over an extended period are all indicators of internal engine wear. Worn piston rings, scored or worn cylinder bores, blown head gaskets, sticking or burnt valves and worn valve seats can all effect engine compression and will show up in a compression test.

This test will provide useful information only if the battery and starter are in good condition and capable of turning the engine at normal cranking speed. As mentioned in General Information And Maintenance , a screw-in type compression gauge is more accurate and the job can be done with only one person.

GASOLINE ENGINES



See Figure 1

  1. Warm up the engine to normal operating temperature.
  2.  
  3. Remove the spark plugs. Note their color and location for later diagnosis.
  4.  
  5. On engines with CIS/CIS-E fuel injection, disconnect the air duct boot from the throttle body. On all models, disconnect the wiring from the primary side of the coil and the Hall sender (if equipped). This will prevent operation of both ignition and fuel injection systems.
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  7. On engines with Digifant fuel injection, disconnect the ignition control unit and the Hall sender. This will prevent operation of both ignition and fuel injection systems.
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  9. Screw the compression gauge into the No. 1 spark plug hole until the fitting is snug.
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  11. Inside the vehicle, depress the accelerator pedal to open the throttle fully and operate the starter continuously for about 3 seconds.
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  13. Record the compression gauge reading, release the pressure and test the remaining cylinders. Be sure to write down each reading.
  14.  



Click image to see an enlarged view

Fig. Fig. 1: A screw-in type compression gauge is more accurate and can be used without the aid of an assistant

  1. Compare the readings to the tune-up chart in Engine Performance And Tune-up . The maximum difference between cylinders must be no more than 44 psi.
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  3. If a cylinder is unusually low, a "wet test" may isolate the problem. Pour about a tablespoon of clean engine oil into the cylinder through the spark plug hole and test that cylinder again.
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  5. Interpret the test results:
    1. If the compression comes up when oil is added, the rings and/or cylinder bore are worn.
    2.  
    3. If compression in any two adjacent cylinders is low, and if the addition of oil doesn't help the compression, there is leakage past the head gasket indicating a warped head.
    4.  
    5. If there is little or no change when oil is added, there is probably a leaking valve or head gasket or even a cracked cylinder head. Look for oil in the coolant, coolant in the oil, signs of coolant in the exhaust system or one spark plug with a significantly different color from the others.
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    7. If all cylinders are low but within the allowable spread, repeat the test with oil in all cylinders. It will probably increase, indicating worn rings.
    8.  

  6.  
  7. Any time the compression test shows a big difference between cylinders, a cylinder leak-down test can provide more information. This requires a special tool with a gauge and fittings to apply air pressure to the cylinder. The rate of cylinder leakage can determine the extent of wear or damage but the only way to really know what's wrong is to remove the cylinder head.
  8.  

DIESEL ENGINES



See Figure 2

Under each injector is a washer to protect the injector tip from heat and form a good seal. These can be used only one time. Have new heat shield washers on hand before removing the injectors.

  1. Carefully remove all the injection lines from the pump and the injectors. Use a back-up wrench when loosening the lines from the pump.
  2.  
  3. Cap all the fittings to prevent dirt from getting into the pump or injectors.
  4.  
  5. Remove the injector leak-off tubes and remove the injectors. Remove the heat shield washers under each injector.
  6.  
  7. Disconnect the fuel shut-off solenoid wire from the injection pump so fuel will not squirt out of the pump during the test.
  8.  
  9. Screw the compression gauge into the No. 1 injector hole until the fitting is snug.
  10.  
  11. Inside the vehicle, operate the starter continuously for about 3 seconds.
  12.  
  13. Record the compression gauge reading, release the pressure and test the remaining cylinders. Be sure to write down each reading.
  14.  
  15. Compare the reading to the tune-up chart in Engine Performance And Tune-up . The maximum difference between cylinders must be no more than 73 psi.
  16.  


CAUTION
Do not add oil to a cylinder when checking diesel engine compression. The compression generates enough heat to ignite the oil in the cylinder.

  1. Except for the added-oil compression test, interpreting compression test results for diesel engines is the same as for gasoline engines.
  2.  



Click image to see an enlarged view

Fig. Fig. 2: Diesel engines require a special compression gauge adaptor

 
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